Various types of scanning arrangements in which comparatively wide band magnetic tape is guided in a spiral path have been proposed. The referenced German Disclosure Document DT-OS No. 1,930,779, the disclosure of which is hereby incorporated by reference, discloses such a scanning arrangement and a method to assemble the scanning structure. In accordance with this disclosure, a drum guide element is provided which guides a composite scanning drum in which a rotating head wheel is located, the head wheel carrying the transducer heads themselves. The attachment guide element for the block is formed with two highly precisely manufactured surfaces which cooperate with the external surfaces of the cylindrical tape guide head. After adjustment, the cylindrical half-elements of the tape head are separably connected to the guide block.
Magnetic tape for use in video tape recording uses a comparatively wide magnetic tape operating at comparatively slow speed, being scanned by one or more heads which operate at an angle with respect to the direction of movement of the tape. The information is thus stored on the tape in the sequence of parallel inclined tracks, carried at an angle with respect to an edge of the tape. An excellent discussion of such equipment is found in the IEE "Spectrum", issue of January 1978 titled "Technology 1978", pages 54, 55, section relating to VCR (video cassette recording). Various types of apparatus have also been proposed in which the magnetic tape is longitudinally deformed, over a minor section thereof to have the shape of the cylindrical surface in which a head wheel runs along the axis, so that the tape is scanned in a direction transverse to its longitudinal movement. The systems in which a tape is spirally guided over a cylindrical guide drum in a spiral path has several advantages. The video recording/playback head which actually causes the inclined track on the tape is guided to rotate in a narrow slit separating two halves of the cylindrical drum. One or more, usually two transducer heads are located on such a head wheel, operating within the slit between the half of the cylindrical guide drum over which the tape is spirally guided. The actual wrapping angle of the tape about the cylindrical head can vary between 90.degree. and almost 360.degree.. The angular position of the separate and tightly adjacent tracks with respect to the center axis of the tape thus can differ. The position and the angle of the tracks should be the same for all apparatus of one type, or model or system with only small tolerances if it is desired to permit interchange of magnetic tape made on one recorder for reproduction on another, or for use or re-recording on another without deterioration of quality. This is particularly important when recording and reproducing television signals in television studios. The accuracy of manufacture and assembly of the scanning system--the cylindrical guiding heads and the head wheel must thus be of highest order. The assembly of the two halves of the cylindrical guiding drum and the tape guide element, which determine the position of the tape on the drum is particularly difficult. The two halves of the guide drum have to be extremely accurate with respect to their geometric dimensions--with tolerances of only a few .mu.m; further, the actual alignment both with respect to the direction of the axis, as well as with respect to the congruence of the axes of the halves must be practically perfect, so that errors of the drum half, after assembly, are essentially eliminated. When the magnetic tape is guided in transit from one half of the drum to the other it should not be subjected to any stresses, or changes in its geometric shape since such changes may result in a deviation of the scanning track from the predetermined ideal track.
The scanning heads or transducers in apparatus of this type, particularly for studio-type video recording/reproducing apparatus are subject to some wear, particularly due to the high scanning speed and the direct engagement of the scanning head with the tape. It is thus necessary to replace the scanning heads after several hundred operating hours, even when using the best heads available according to the present State of the Art. It is thus desirable that the head wheel operating in the separating gap or slit of the guide drum can be readily replaced after separating the guide drum halves from each other and that, after reassembly of the entire scanning system, the same accuracy of placement can be obtained as that originally provided. This reproduceability of location should be easily obtained and not require expensive machining, yet have the same positioning accuracy as the initial assembly of the elements with respect to each other.